ThermoWood Birch Thermo-D

About ThermoWood Birch Thermo-D

ThermoWood Birch Thermo-D is Nordic birch (Betula) modified at about 212°C. At 1,210 lbf (5,382 N) and 0.62 g/cm³ it sits just below Thermo Ash, and it has the finest, most even texture of anything in the range. Birch has almost no figure and very few knots, so what you get after treatment is a plain, warm, uniform brown surface.

Note: this is a modified product, and the panel above is modelled rather than measured. The hardness figure is the base species, because thermal modification changes hardness very little. For movement, the shrinkage and the fibre saturation point both fall after treatment, and that is why modified timber moves less across the same change in moisture. Published strength reductions vary by source; the table above uses our library figures for this pair. Modification also lowers bending strength and toughness relative to the untreated timber. Use this panel to size and to compare species, and take structural, fire, durability or movement figures from your supplier's declaration for the product you are actually buying.

That evenness is the selling point. Where ash and oak bring grain and pore structure into the room, birch brings none, which is why it turns up in sauna panelling and in interiors that want the colour without the pattern. Radial shrinkage of 1.6% and tangential of 3% are about a third of natural birch, a timber otherwise notorious for moving.

At 620 kg/m³ a cubic metre of ThermoWood Birch Thermo-D weighs about 620 kg air-dry — that figure, the 3.0% tangential and 1.6% radial movement and the 1,210 lbf hardness are the three inputs the calculators below actually consume. Select ThermoWood Birch Thermo-D in any of them to get species-specific results for weight, moisture, shrinkage and hardness.

Working properties

Birch machines beautifully and always has. The fine, even, diffuse-porous structure that makes it the standard plywood face veneer also makes it cut cleanly in every direction. How the board behaves once it is out of the kiln is set by the process itself, which is described in full in our guide to how ThermoWood is made. What follows is specific to this timber, drawn from the International ThermoWood Association, VTT Technical Research Centre of Finland and published thermal-modification research.

Durability & use classification

Birch is in Table 1 of the ITWA ThermoWood Handbook for both treatment classes, so genuine marked ThermoWood Birch exists. As a Thermo-D product, treated near 212°C, ITWA classifies it as durable, Class 2 under EN 113. Untreated birch is not durable at all and rots quickly outdoors, so this is another large jump bought entirely with heat. The durability is a property of the treatment, not of the heartwood, and it goes through the board, so there is nothing to trim. ITWA states ThermoWood is not intended for load-bearing structures, and the name is a trademark; CEN/TS 15679 calls the generic material Thermally Modified Timber. See our guide to how ThermoWood is made.

In practice Thermo Birch is mostly an interior product. It can go outside, and Class 2 says it will last, but the reason people choose birch over pine is the fine surface, and that surface argument is an indoor argument. Saunas, panelling, ceilings and furniture components are where it earns its price.

Typical applications & suitability

Indoor Outdoor — Like ash, birch is naturally non-durable, so modification is what allows it outdoors. It also darkens the pale, uniform birch surface to a rich brown right through the section — a large part of the appeal.

Documented end-uses: exterior cladding, interior panelling and flooring, sauna benches and interiors, furniture and joinery.

Comparable species

Species closest to ThermoWood Birch Thermo-D by density, hardness, stiffness and bending strength, computed directly from the Vriksai species dataset rather than hand-picked.

Trade alternatives. The untreated originals are Silver Birch and Yellow Birch — both interior-only. Compare ThermoWood Ash.

What modification did to Silver Birch

ThermoWood Birch Thermo-D is not a species — it is Silver Birch after heat treatment. Both sets of figures are in our library, so the effect of the process on this timber can be stated exactly rather than described in general terms.

On this species the process buys a 62% cut in tangential movement and a 3-class gain in durability, paid for with 17% of its bending strength. A normal trade-off for this treatment class: fine for cladding and exterior joinery, size structural work off the treated figure, never the untreated one.

The mechanism — hemicellulose breakdown, why durability is through-section rather than surface-deep, and why the strength loss happens — is set out once in our guide to thermal modification and the ThermoWood species reference, rather than repeated on each treated species page. Untreated figures: Silver Birch.

Compare ThermoWood Birch Thermo-D side by side

Modified against unmodified, and modification class against class — the comparisons that matter for treated timber. Every pairing shows the measured figures side by side; full matrix in the comparison index.

Calculators relevant to ThermoWood Birch Thermo-D

Includes the thermal-modification tools, since ThermoWood Birch Thermo-D behaves by treatment class as much as by species. Full set in the tool index.

Understand the figures

Strength properties — MOR, MOE and what they govern · Durability & fire — EN 350 classes and E84 ratings · Thermal conductivity — k-value and insulation behaviour · ThermoWood species guide — Thermo-D/Thermo-S treatment classes.

Further reading. What is ThermoWood? · ThermoWood vs Teak · Wood moisture content explained.